Nowadays, the energy efficiency and the environment protection become more and more important. The expectation of the standby efficiency of a power supply is also increased. The power supplies are requested to satisfy many green energy standards, such as BLUE ANGEL, ENGERY STAR, ENERGY 2000, etc. As a result, a pulse skipping mode is generated to enhance the standby efficiency. During the pulse skipping mode, some switching pulses are skipped to reduce the switching loss while the regulation of the output voltage is maintained.
Generally, a feedback signal representative of the output voltage of the power supply is compared with a reference signal by an error amplifier, so as to generate a compensation signal. The compensation signal is compared with a constant threshold voltage hysteretically. When the compensation signal is smaller than the constant threshold voltage, one or more switching pulses of the control signal are skipped to keep the switch off until the compensation signal becomes larger than the constant threshold voltage. Ideally, during the pulse skipping mode, there is only one switching pulse at a time. The time period between two adjacent switching pulses is determined by the load. The lighter the load, the longer the time period, and vice versa.
However, under the influence of the output filter and the error amplifier, the power supply usually needs two or more switching pulses to reduce the compensation signal to be smaller than the constant threshold voltage. The efficiency and stability of the power supply is limited, and the ripple of the output voltage is large.